Unless we can find out what Intamin's recommended maintenance windows are for this ride, and what Alton were actually doing, then there's no way we can say it was the park's fault.
As for this "there were people checking the cable last week and they evacuated the queue and everything" I don't really see how that's necessarily relevant.
1) How do you know it was the actual cable they were checking? It could've been one of the other bits of equipment in the same area of the ride.
2) For all I (and I'd guess you too) know, it could be policy to check the cable every time there's an unidentified/non-routine/slightly larger problem with the ride. It might not be, but unless we know for sure, it's possible.
3) It's common for Merlin parks to evac the queue if it's obviously going to take a while to fix, or if they've already spent a while, and it's not likely to re-open any time soon. I'm guessing this may even be company policy, but never having worked there, I wouldn't know. Just because they evac the queue doesn't make it a "serious" problem it just makes it a "longer to fix than a re-set" problem.
Anyway, without spending the rest of the night googling stuff, the best information I've already found relates to the cable snap on Xcelerator in 2009. Now, clearly, that's a long time ago, on an older ride, so things are likely to have changed, but like I say, it's the best I could find easily. One of the issues was that Intamin's instructions on how often to do a "full" cable check were proven to be unclear. The park interpreted it as once ever 6 months, but Intamin claimed that they had intended it to be done once a month. Anyway, it turned out it had been 6 months and 19 days since the last big inspection, so Knott's were in the **** regardless. There were also visual checks done daily, and a full cable replacement once a year.
Assuming (and, as I say, this is unlikely) that the parameters are the same for Rita in 2012, then they'd be looking at a daily visual check, a monthly "full" check, and a scheduled annual replacement. I have no reason to believe that even Merlin would be stupid enough to not follow the guidelines, as they would leave themselves far too open to lawsuits.